New York, July 9 (IANS) Social media giant Facebook has said that it has begun offering an option to encrypt messages -- a way to have secret conversations with other users -- to some of 900 million users of Messenger, its messaging app.

The end-to-end encryption is a system that allows only the communicating users to read the messages. The upgrade will prohibit even Facebook from reading users' conversation.

"We want people to be able to use Messenger for every conversation that they want to have," The Wall Street Journal quoted Tony Leach, product manager for Facebook Messenger as saying.

"In some cases, these are really fun and casual things like sending emojis to one another. In other cases, these are really serious conversations where you need an extra layer of security," Leach added.

Facebook introduced end-to-end encryption to one billion users of WhatsApp messaging app in April.

Leach said encrypting Messenger is a tougher challenge than WhatsApp because the service is widely used via browsers on personal computers, as well as on smartphones.

While on WhatsApp, the end-to-end encryption is turned on by default, with Messenger users will have to turn on the feature, which hasn't gone down well with a few.

Christopher Soghoian, a principal technologist with the American Civil Liberties Union, wrote in a Twitter post that "opt-in encryption favours educated users who have the time to learn about obscure security settings".

Meanwhile, Matthew Green, an assistant professor of computer science at Johns Hopkins University, who was invited by Facebook to review its end-to-end encryption effort, said he would like to see the feature turned on by default but agreed that this would be a challenge.

"It is really hard to get an encrypted messaging app that has all the features that Facebook wants to work on the web and on multiple devices," Green noted.

Facebook said end-to-end encryption would not immediately be available on Messenger for group conversations, chatbots, payments or videos.​